For example, lane markings are provided for designating vehicle lanes on a road surface. There is known a lane marking recognition apparatus for recognizing such lane markings in an image of the road captured by a vehicle-installed forward-imaging camera (see, for example, JP-2007-018154-A). For example, a single solid-state image capture element which generates Bayer pattern array gray-scale image data (called RAW data) may be used as a vehicle-installed camera in a lane marking recognition apparatus. For the RAW data, demosaicing processing is performed to estimate/interpolate unavailable red values (R), green values (G) and blue values (B) for each pixel, thereby generating RGB color image data (see, for example, U.S. Pat. No. 5,506,619-B, U.S. Pat. No. 5,629,734-B and U.S. Pat. No. 5,652,621-B). Such demosaicing process will be performed in the image capture apparatus or in the following-stage image processing apparatus. The lane marking recognition apparatus of JP-2007-018154-A also includes image acquisition means for performing demosaicing processing on a video signal supplied from a color video camera.
Recently Quad-VGA size (1280 pixel×960 pixel) and SXGA size (1280 pixel×1024 pixel) CMOS imaging sensors have become relatively low cost and are becoming more common that, for example, perform demosaicing processing on RAW data from a Quad-VGA size Bayer pattern array, reducing the frame size to VGA size (640 pixel×480 pixel). Such frame-size-reducing demosaicing processing is referred to below as resolution degradation demosaicing processing.
Brief explanation follows regarding an example of related-art resolution degradation demosaicing processing. FIGS. 7A and 7B illustrate, respectively, base Bayer pattern array RAW data and primary RGB color image data resulting from resolution degradation demosaicing processing this RAW data. In FIG. 7A, pixels R are red pixels, pixels B are blue pixels, and pixels Gr and pixels Gb are green pixels. The pixels Gr are located within R-pixel lines in the X direction (horizontal direction), and the pixels Gb are located within B-pixel lines in the X direction. In FIG. 7A, the 2×2 pixel array containing the top-left pixel Gr, the top-right pixel R, the bottom-left pixel B and the bottom-right pixel Gb is a base pixel pattern.
In FIG. 7B, the pixels Pij (where i and j are positive integers) are pixels after resolution degradation demosaicing processing.
The pixel CP in FIG. 7B is the pixel for the base pixel pattern BP of FIG. 7A after resolution degradation demosaicing processing. In this resolution degradation demosaicing processing, for example, each of the values of the pixel R and the pixel B of the base pixel pattern BP is taken as the red value and blue value of the pixel CP, respectively, and the average value of the values of the pixel Gr and the pixel Gb of the base pixel pattern BP is taken as the green value of the pixel CP.
Namely, by performing the resolution degradation demosaicing processing, reduced-size color image data is generated from the RAW data to be half the frame size in both the X direction and the Y direction.
However, depending on the pattern of the image, false color is sometimes generated in the color image data after the resolution degradation demosaicing processing as described above. For example, in a lane marking recognition apparatus, image data of a road surface may be captured by an image capture apparatus, and the resolution degradation demosaicing processing may be performed on the captured image data. In this case, false yellow color will be generated in the color image data for the image patterns, as shown in FIGS. 8A to 8C. FIG. 8A exemplarily illustrates a Bayer pattern array for a diagonal white line, FIG. 8B exemplarily illustrates a Bayer pattern array for a vertical white line, and FIG. 8C exemplarily illustrates a Bayer pattern array for a horizontal white line. In all cases, the brightness of pixels corresponding to the road surface other than at the white line is lower than the brightness of pixels corresponding to the white line. In such image pattern, when resolution degradation demosaicing processing is performed on a base pixel pattern having a large R pixel value and a small B pixel value, due to the red pixel R and the green pixel Gr/Gb, a yellow component that does not really exist is generated. Namely, false yellow color is generated. In each of the examples shown in FIGS. 8A to 8C a yellow line is generated at the edge portion of the white line (the portions inside the bold lines).
Such false yellow color can also be generated by portions where white lines on the road surface are wearing/flaking or cracks have developed in the white lines due to the passage of vehicles. False yellow color also readily occurs around edges of the bodies of white vehicles traveling on the road.
There are also other lane markings on the road surface in addition to white lines, such as yellow lines, transverse lines, Bott's dots, and cat's eyes. Bott's dots are circular plates of about 10 cm diameter with a raised height of 1 cm, and plural Bott's dots are provided on vehicle lane dividing lines or on hypothetical lines where vehicle lanes should be divided. Cat's eyes are substantially rectangular shaped reflective bodies, with plural cat's eyes provided for dividing vehicle lanes.
When a lane marking recognition apparatus employs color image data generated by the related-art resolution degradation demosaicing processing for recognizing lane markings, false yellow color may be present in the color image data, and false-positive recognition and/or a degradation in lane marking detection precision may occur.